With the recent trend toward the thinning and upsizing of a liquid crystal display device, the thickness of the components used therefor becomes increasingly thinner. The liquid crystal display device is equipped with a backlight that is a luminous body portion emitting light behind the LCD, and a light guide plate and a diffusion plate are used to diffuse or transfer light, depending on the type or position of the light source. The thickness of the light guide plate becomes increasingly thinner in accordance with the recent trend. For the general level of the light guide plate that is actually used, the thickness is around 0.5 mm, but the thinnest is up to about 0.3 mm, and the thickness tends to be thinner in the future.
Instead of a cold cathode fluorescent lamp (CCFL) mainly used in accordance with the thinning trend, an edge-type backlight unit equipped with LED in the edge portion of the backlight has been increasingly used. The edge-type backlight unit enables light started from a light source equipped in the edge portion to transmit through a light guide plate, and part of the light transmitted through the plate is scattered by the light scattering layer applied to the surface of the plate, thereby lightening a liquid crystal display device by a surface light source where the whole surface uniformly emits light. Such light scattering layer is formed by transferring or printing a dot pattern on the surface of the light guide plate, and recently, a prism structure of a microstructure may be transferred in order to increase the light efficiency.
Since the light guide plate requires a high light transmittance, PMMA, which is an acryl-based resin, has generally been used as the material of the light guide plate. The acryl-based resin has a high light transmittance, but it has insufficient mechanical strength and thus is not suitable to apply to a thin light guide plate, and them is a disadvantage that it is vulnerable to heat generated from electronic devices, due to low heat resistance thereof.
Polycarbonate has attracted attention in place of such acryl-based resin. The polycarbonate has excellent mechanical strength as compared with the acrylic resin and thus can be used as a material of a thin light guide plate. Also, the polycarbonate has excellent heat resistance, flame retardancy and thus is gradually replacing the acrylic resin in LED-applied backlight unit and lighting apparatus with a large heating value. However, since the polycarbonate has a low light transmittance as compared with the acrylic resin, there is a need to have a light transmittance corresponding to the acrylic resin while maintaining the advantages of the polycarbonate.
In this regard, Japanese Patent Application Publication No. 2008-045131 has suggested that among acryl-based resins, PMMA having a viscosity average molecular weight (Mv) of 20,000 to 60,000 is blended with polycarbonate having Mv of 15,000 to 40,000 to achieve an excellent light conductivity. However, there is still a need to improve physical properties such as heat resistance.
Thus, the present inventors have conducted numerous studies to develop the materials that can be used as a material of a light guide plate, and found that, as described below, a resin composition comprising polycarbonate and polyoxytetramethylene-polyoxypropylene glycol random copolymer is excellent in light transmittance, heat resistance and the like and thus can be used as a material of the light guide plate, thereby completing the present invention.